To resolve this problem, follow these steps:
- Apply the hotfix that is mentioned in the following Microsoft Knowledge Base article: 
956925 Description of the ISA Server 2006 hotfix package: August 20, 2008
  - Start Notepad, and then copy the following script into a Notepad file.
Const SE_VPS_GUID = "{143F5698-103B-12D4-FF34-1F34767DEabc}"
Const SE_VPS_NAME = "EnableAuthZNULLContextCaching"
Const SE_VPS_VALUE = true
Sub SetValue()
    ' Create the root obect.
    Dim root  ' The FPCLib.FPC root object
    Set root = CreateObject("FPC.Root")
    'Declare the other objects needed.
    Dim array       ' An FPCArray object
    Dim VendorSets  ' An FPCVendorParametersSets collection
    Dim VendorSet   ' An FPCVendorParametersSet object
    ' Get references to the array object
    ' and the network rules collection.
    Set array = root.GetContainingArray
    Set VendorSets = array.VendorParametersSets
    On Error Resume Next
    Set VendorSet = VendorSets.Item( SE_VPS_GUID )
    If Err.Number <> 0 Then
        Err.Clear
        ' Add the item
        Set VendorSet = VendorSets.Add( SE_VPS_GUID )
        CheckError
        WScript.Echo "New VendorSet added... " & VendorSet.Name
    Else
        WScript.Echo "Existing VendorSet found... value- " &  VendorSet.Value(SE_VPS_NAME)
    End If
    if VendorSet.Value(SE_VPS_NAME) <> SE_VPS_VALUE Then
        Err.Clear
        VendorSet.Value(SE_VPS_NAME) = SE_VPS_VALUE
        If Err.Number <> 0 Then
            CheckError
        Else
            VendorSets.Save false, true
            CheckError
            If Err.Number = 0 Then
                WScript.Echo "Done with " & SE_VPS_NAME & ", saved!"
            End If
        End If
    Else
        WScript.Echo "Done with " & SE_VPS_NAME & ", no change!"
    End If
End Sub
Sub CheckError()
    If Err.Number <> 0 Then
        WScript.Echo "An error occurred: 0x" & Hex(Err.Number) & " " & Err.Description
        Err.Clear
    End If
End Sub
SetValue
 - Save the file as a Microsoft Visual Basic script file by using the .vbs file name extension. For example, save the file by using the following name: 
EnableAuthZNULLContextCaching.vbs
 - Copy the .vbs file to the computer that is running ISA Server 2006, and then double-click the file. 
 
After you apply these steps, ISA Server 2006 caches the negative responses from the 
AuthZ API and only tries to create the security context one time for each user.